The present invention is directed to walk-off compensation in optical frequency conversion systems such as parametric oscillators, second harmonic generators, sum frequency generators, and difference frequency generators.
The use of various crystals in critically phasematched, three-wave optical mixers is well known, and is exemplified in known optical second harmonic generators, (SHG), sum frequency generators (SFG), difference frequency generators (DFG), and optical parametric oscillators (OPO). Such systems typically utilize crystals such as BaB.sub.2 O.sub.4, LiNbO.sub.3, LiIO.sub.3, KTiOPO.sub.4, LiB.sub.3 O.sub.5, KDP, and others which correspond to optical inputs to provide frequency mixing of the signals. However, it has been found that the conversion efficiency of such mixing systems are often limited by walkoff of optical signals.
The problem of walkoff is illustrated in an optical parametric oscillator utilizing a BaB.sub.2 O.sub.4 crystal for frequency conversion. The development of broadly tunable, high power, coherent radiation sources based on the optical parametric oscillator (OPO) has accelerated recently due to the availability of excellent new materials, such as low temperature phase barium metaborate (BBO), or .beta.-Ba B.sub.2 O.sub.4. Such materials are described, for example, by C. Chen, Y. X. Fan, R. C. Eckardt, and R. L. Byer in "Recent Developments in Barium Borate", Proceedings SPIE, Vol 681, pp. 12-19, 1986. The ability of BBO to phase match in the ultraviolet, in addition to having a broad transmission range, a high ultraviolet damage threshold, and a high nonlinearity have established this material as an excellent choice for parametric conversion in the ultraviolet, visible, and near infra-red wavelengths. Optical parametric oscillators pumped at 266 nm, 308 nm, 355 nm, and 532 nm, have been reported to collectively generate continuously tunable radiation from 0.33 micrometers to 3.1 micrometers, with conversion efficiencies ranging from a few percent to 32 percent. Thus, the optical parametric oscillator utilizing BBO has established itself as an excellent source of broadly tunable, pulsed radiation, as described in copending application Ser. No. 07/379,781, to Chen et al, filed Jul. 14, 1989 assigned to the assignee herein.
BBO crystals have a relatively large birefringence which enables phase matching very close to their ultraviolet cutoff at 190 nm. However, in the critically phase matched BBO optical parametric oscillator, this leads to a large walkoff angle between the extraordinary pumping beam and the ordinary signal and idler beams which limits conversion efficiency by reducing the effective interaction length in the crystal. Similar walkoff problems exist in other critically phasematched three-wave conversion systems.